In dentistry, various dental restorative materials are used for restorating a deficit after medically treating a decay in a natural tooth etc. Among these are dental restorative materials prepared by coating and firing a dental alloy substrate as a coping. Such materials are prepared by a casting method, where a dental porcelain is fused to a metal material at 800.degree. C. to 1,000.degree. C. (the dental restorative material may hereinafter be referred to as "dental porcelain fused to metal crown"). This material is not only chemically stable but also has translucency and color tone well matched with a natural tooth and hence has been widely used.
However, when such a dental restorative material is set up and fixed in the mouth and it is applied with external forces, for example, by mastication, etc., its coating often breaks. In order to solve this problem, various attempts have been made to reduce the external forces applied to the coated material by altering its form. In order to give a natural feeling when set up in the mouth or to meet the aesthetic sense of a person who sets it up, it is desired that the strength of the material itself (the coated material) of the dental restorative material is improved to thereby prevent the breakage caused by external forces, without affecting its natural form. From this viewpoint, various modifications of the composition of the dental porcelain fused to a metal material used as a raw material of the above described coated material have been made but not without satisfactory effects. For example, when a crystal of high purity alumina is contained in a dental porcelain fused to metal material, the strength is improved, but at the same time opacity increases and the color tone, as in a natural tooth, is lost. Accordingly, the development of a method for thoroughly strengthening the coated material of the dental restorative material while maintaining translucency and color tone (as in a natural tooth), without taking an unnatural form has still been sorely required in the field of dental restoration.
In order to meet this demand, the inventors have extensively investigated and finally confirmed that when, after forming a coated material, the coated material is deposited with a specified inorganic salt of metal added thereto and then heated at a specified temperature, the coated material can be thoroughly strengthened by inorganic salt at a non-molten state.